1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an IPS mode liquid crystal display device.
2. Discussion of the Related Art
From among very thin flat panel display devices having screens with a thickness of only a few centimeters, a liquid crystal display device is mainly used in wide variety of fields such as a laptop computer, a monitor, and aircraft.
The liquid crystal display includes a lower substrate, an upper substrate, and a liquid crystal layer formed between the upper and lower substrates. In the liquid crystal display device, when voltage is applied, an arrangement of the liquid crystal is changed and transmittance of light based upon the rearranged liquid crystals is adjusted to display images.
Hereinafter, a related art liquid crystal display device will be described in detail with reference to the accompanying drawings.
FIG. 1 is a schematic perspective view illustrating a related art liquid crystal display device, in particular, a twisted nematic (TN) mode liquid crystal display device. FIG. 1A illustrates a state of the related art TN mode liquid crystal display device to which voltage is not applied, and FIG. 1B illustrates a state of the related art TN mode liquid crystal display device to which voltage is applied.
First, the structure of the related art TN mode liquid crystal display device will be described briefly and the operation principle thereof will be described in detail.
The Related Art TN mode liquid crystal display device includes a first substrate 1, a second substrate 3, and a liquid crystal layer 5 formed between the first and second substrates 1 and 3.
The first substrate 1 includes a first polarizing plate 7 formed on the outer side thereof and having a transmission axis oriented to a predetermined direction, and the second substrate 3 includes a second polarizing plate 9 formed on the outer side of the second substrate 3 and having a transmission axis opposite to that of the first polarizing plate 7.
Moreover, although not depicted in the drawings, the first substrate 1 includes a pixel electrode and the second substrate 3 includes a common electrode so that a vertical directional electric field is generated between the pixel electrode and the common electrode.
As shown in FIG. 1A, when voltage is not applied, the liquid crystal layer 5 is twisted by 90 degrees between the first substrate 1 and the second substrate 3. Here, when light 10 enters through the second polarizing plate 9, the light 10 passes through the liquid crystal layer 5. At this time, like molecules of the liquid crystal layer twisted by 90 degrees, the light 10 is twisted by 90 degrees and passes through the first polarizing plate 7. Thus, a white image is displayed.
As shown in FIG. 1B, when voltage is applied, the molecules of the liquid crystal layer 5 are vertically arranged between the substrates 1 and 3 by the vertical direction electric field between the pixel electrode and the common electrode. Here, when the light 10 enters through the second polarizing plate 9, the light 10 passes through the liquid crystal layer 5. At this time, since the polarizing direction of the light does not rotate, the light 10 cannot pass through the first polarizing plate 7. Thus, a black image is displayed.
However, the TN mode liquid crystal display device has a critical shortcoming of having a narrow viewing angle.
FIGS. 2A and 2B are views illustrating the problem of the viewing angle of the related art TN mode liquid crystal display device.
FIG. 2A illustrates the white displaying state in which voltage is not applied, FIG. 2B illustrates the black displaying state in which full voltage is applied, and FIG. 2B illustrates the intermediate displaying state in which intermediate voltage is applied.
As shown in FIG. 2A, when voltage is not applied, the molecules of the liquid crystal layer 5 are twisted by a minute angle in the same direction and the incident light (depicted by arrows in the drawing) is displayed as white light in all directions.
As shown in FIG. 2B, when the full voltage is applied, the molecules of the liquid crystal layer 5 are arranged in the vertical direction due to the effect of the electric field and the incident light is not twisted and therefore black is displayed.
As shown in FIG. 2C, when the intermediate voltage is applied, the molecules of the liquid crystal layer 5 are arranged in the slant direction and the displayed states are different according to the directions of the incident light. In other words, a light entered in the direction from right lower side to the left upper side displays black because the polarizing direction of the light is not changed, and on the contrary, a light entered in the direction from the left lower side to the right upper side is displayed as white because the polarizing direction of the light is twisted.
As such, the related TN mode liquid crystal display device has the shortcoming such that a displaying state is different based upon the incident angle of the light and the viewing angle is narrow.
Research for widening the viewing angle is being carried out with vigor and various approaches are proposed. For example, there are proposed an in-plane switching (IPS) mode using a horizontal directional electric field, an vertical alignment (VA) mode using a vertical arranged layer, and an electrically controlled birefringence (ECB) mode, a multi-domain method of dividing a domain and using a mean value of the arrangement of the molecules of the liquid crystal layer, and a phase compensation method of using a phase difference film to change phase difference according to the change of the viewing angle.
The present invention relates to an IPS mode liquid crystal display device among the methods of widening a viewing angle, and hereinafter the related art IPS mode liquid crystal display device will be described in detail.
FIGS. 3A-3D illustrate the related art IPS mode liquid crystal display device, wherein FIGS. 3a and 3b are a sectional view and a plan view, respectively, illustrating the related art IPS mode liquid crystal display device to which voltage is not applied, and FIGS. 3c and 3d are a sectional view and a plan view, respectively, illustrating the related art IPS mode liquid crystal display device to which voltage is applied.
First, the structure of the related art IPS mode liquid crystal display device will be briefly described and the operation principle thereof will be described in detail.
The related art IPS mode liquid crystal display device includes a first substrate 1, a second substrate 3, and a liquid crystal layer 5 formed between the first and second substrates 1 and 3.
The first substrate 1 includes a first polarizing plate 7 formed on the outer side thereof and having a transmission axis oriented to a predetermined direction, and the second substrate 3 includes a second polarizing plate 9 formed on the outer side of the second substrate 3 and having a transmission axis opposite to that of the first polarizing plate 7.
Moreover, the first substrate 1 includes a pixel electrode 2 and a common electrode 4 parallel to each other such that a horizontal directional electric field is generated between the pixel electrode 2 and the common electrode 4.
As shown in FIGS. 3A and 3B, when voltage is not applied, the liquid crystal layer 5 is arranged between the first substrate 1 and the second substrate 3 approximately parallel to the longitudinal direction of the electrodes 2 and 4. Here, when light 10 enters through the first polarizing plate 7, the light 10 transmits to the liquid crystal layer 5. At this time, since the polarizing direction of the light 10 does not rotate, the light 10 cannot pass through the second polarizing plate 9 having the transmission axis opposite to the transmission axis of the first polarizing plate 7. Thus, a black image is displayed.
As shown in FIGS. 3C and 3D, when voltage is applied, the liquid crystal layer 5 is arranged differently in the vicinity between the first substrate 1 and in the vicinity of the second substrate 3. In other words, the liquid crystal layer 5 is arranged in the vicinity of the first substrate 1 in the vertical direction with respect to a longitudinal direction of the electrodes 2 and 4 due to the horizontal directional electric field between the pixel electrode 2 and the common electrode 4, and is arranged in the vicinity of the second substrate 3 in the horizontal direction with respect to the longitudinal direction of the electrodes 2 and 4, like the case when the voltage is not applied because the effect of the electric field is weak.
Thus, when the light 10 enters through the first polarizing plate 7, the light 10 then passes through the liquid crystal layer 5. At this time, since, like the molecules of the liquid crystal layer 5, the light 10 is twisted and passed through, the light 10 passes through the second polarizing plate 9 opposite to the first polarizing plate 7. Thus, a white image is displayed.
As such, since, in the related IPS mode liquid crystal display device, the molecules of the liquid crystal layer are erected vertically but switched horizontally, the viewing angle is not changed according to the incident angle of the light even when the intermediate voltage is applied.
However, since the pixel electrode 2 and the common electrode 4 are formed on the first substrate 1, the related IPS mode liquid crystal display device has a low transmittance.
The reason of the low transmittance is because the aperture ratio is decreased as much as the widths of the pixel electrode 2 and the common electrode 4. At the present time, the widths of the pixel electrode 2 and the common electrode 4 is about 4 μm, which is a minimum width capable of being formed by photolithography. When the photolithography is carried out, there is a limit to reduction in the widths of the pixel electrode 2 and the common electrode 4 due to the characteristics of photolithography.